ALBERT

Description: The objectives of this program are as follows: modelling of dynamics of composite tubular space structure truss members; and utilization of ultrasonic waves as probes for material and defect characterization. This discussion is presented in viewgraph format.

Description: In the interest of aviation safety, NASA and the FAA are jointly conducting research to determine the applicability of airborne, coherent Doppler radar techniques to detect early microburst in wind shear conditions during aircraft takeoff and landing. Researchers have developed a computer model of the radar which predicts its response when viewing a simulated microburst against the simulated clutter background of an airport, the so-called radar microburst ground clutter model. Studies employing this model revealed that Doppler radar can accurately detect microburst ahead of the aircraft in time for pilot evasive response, but flight experiments will be required for complete performance evaluation of the system. An experimental X band radar is being developed for future flight experiments to verify the simulation modeling results. A description of the experimental radar, recording equipment, and its installation on the NASA 515 aircraft is presented. The flight experiments to be conducted are also described.

Description: A new technique is described for calibration of complex multipolarization SAR imagery. Scatterer reciprocity and lack of correlation between like- and cross-polarized radar echoes for natural targets are used to remove cross-polarized contamination in the radar data channels without the use of known ground targets. If known targets are available, all data channels can be calibrated relative to one another and absolutely as well. The method is verified with airborne SAR data.

Description: The SAR (synthetic-aperture radar) data-processing algorithm to be used for the Magellan mission is described. Radar system design, SAR data characteristics, and hardware (H/W) constraints, which are critical to the processing algorithm design, are highlighted. Data flow and the H/W architecture are given to show the real-time data processing capability. Simulation results obtained from processing the synthetic point-target echos are presented to demonstrate the performance of the processing algorithm.

Description: The analysis and low-power testing of a square multiflare horn designed for 1-megawatt CW operation is described. Design considerations for the five-port input section are discussed. The required aperture modes are determined from radiation pattern considerations, and analysis of the multiflare section is carried out using mode matching. Measurements demonstrate that a circularly symmetric beam is produced with relatively low sidelobes.

Description: The cross polarization characteristics of rectantular patch antennas are studied experimentally. Data are presented showing the dependence of the copolarization to cross-polarization ratio on the aspect ratio in both the E and H planes. Three substrate thicknesses are included and the variation with resonant frequency is examined.

Description: Synthesis of dual-shaped offset reflector antennas to control the exit aperture distribution of amplitude and phase has received considerable attention in recent years. For a given feed illumination and desired aperture field distribution, an exact formulation of the problem of simultaneously synthesizing the shapes of the sub and main reflectors was presented recently by Galindo-Israel et al. (1987) in terms of a set of nonlinear first-order differential equations. In this paper, a numerical approach to solving these equations is discussed which circumvents some of the difficulties encountered by Galindo-Israel et al., particularly for small values of theta.

Description: The technique described uses the radar return from natural targets and at least one trihedral corner reflector to calibrate compressed polarimetric radar data for relative amplitude, relative phase, absolute amplitude, and system crosstalk. The crosstalk method is based on the theoretical result that, for natural targets with azimuthal symmetry, the copolarized and cross-polarized components of the scattering matrix are uncorrelated. This method does not require any external calibration targets to be deployed. Since compressed data are used, it is necessary to model the transmitting and receiving systems as reciprocal. The method is used to estimate the crosstalk parameters of the NASA/JPL aircraft for different types of terrain and for two frequencies. For the C-band systems the crosstalk is less than -20 dB for all ranges in the images. The crosstalk of the L-band system is a function of range, however, and may be as poor as -10 dB in the near range, leading to a noticeable distortion of the polarization signatures.

Description: A technique for estimation of the Doppler centroid of an SAR in the presence of large uncertainty in antenna boresight pointing is described. Also investigated is the image degradation resulting from data processing that uses an ambiguous centroid. Two approaches for resolving ambiguities in Doppler centroid estimation (DCE) are presented: the range cross-correlation technique and the multiple-PRF (pulse repetition frequency) technique. Because other design factors control the PRF selection for SAR, a generalized algorithm is derived for PRFs not containing a common divisor. An example using the SIR-C parameters illustrates that this algorithm is capable of resolving the C-band DCE ambiguities for antenna pointing uncertainties of about 2-3 deg.

Description: A theoretical analysis of polarization filtering for the bistatic case is developed for optimum discrimination between two types of targets. The resulting method is half analytical and half numerical. Because it is based on the Stokes matrix representation, the targets of interest can be extended targets. The scattered field from such targets is partially polarized. This method is then applied to the monostatic case with numerical examples relying on the JPL (Jet Propulsion Laboratory) full-polarimetric L-band radar data. A matched filter to maximize the power ratio between urban and natural targets is developed. The results show that the same filter is optimal for both ocean and forest targets as natural targets.

Description: The unique characteristics of a spaceborne SAR (synthetic aperture radar) operating in a squint mode include large range walk and large variation in the Doppler centroid as a function of range. A pointing control technique to reduce the Doppler drift and a new processing algorithm to accommodate large range walk are presented. Simulations of the new algorithm for squint angles up to 20 deg and look angles up to 44 deg for the Earth Observing System (Eos) L-band SAR configuration demonstrate that it is capable of maintaining the resolution broadening within 20 percent and the ISLR within a fraction of a decibel of the theoretical value.

Description: The history of the Controlled Ecological Life Support System program, initiated by NASA in the late 1970s to explore the use of bioregenerative methods of life support, is reviewed. The project focused on examining the process involved in converting inorganic minerals and gases into life support materials using sunlight as the primary energy source. The research, planning, and technological development required by the CELSS program and conducted at NASA field centers, at various universities, and by commercial organizations are reviewed. Research activities at universities have focused upon exploring methods of reducing the size of the system, reducing system power requirements, understanding issues that are associated with its long-term stability, and identifying new technologies that might be useful in improving its efficiency. Research activities at Ames research center have focused on the use of common duckweed as a high biomass-producing plant, which is high in protein and on waste processing.

Description: The bioregenerative life support systems research at Johnson Space Center focuses on the use of lunar regolith as a plant growth medium. Current dissolution experiments are being conducted to ascertain the response of lunar regolith to various solvents and weathering environments. The transformation of lunar minerals into minerals such as zeolites which would be more conducive to plant growth is also investigated. A study is currently underway to examine the ability of zeolite/apatite mixtures to provide N, P, and K through dissolution and ion exchange. The development and characterization of simulated lunar regolith for preliminary experimentation are also discussed. The life support systems technology used on the Mercury, Gemini, Apollo, and Shuttle missions is reviewed and current research on regenerative life support systems technology for potential use in Space Station Freedom is discussed.

Description: The CELSS Breadboard Project is described, noting that it was initiated to study aspects of a CELSS for long-term space missions. Topics for extensive investigation included air and water regeneration, engineering control, and food production. The many options available for growing food crops in commercial plant growth chambers were investigated and the best of this information was translated to the Biomass Production Chamber (BPC). The chamber contains 20 sq m of crop growing area under 96 400 W HPS lamps; sixteen 0.25 sq m plant growth trays used on each of four growing shelves for a total of 64 trays; and one 256-L nutrient solution reservoir with the appropriate continuous-flow, thin-film plumbing for each shelf. A heating, ventilating, and air-conditioning system maintains atmospheric conditions and serves to distribute oxygen and carbon dioxide and maintain pressure at 12 mm of water. The control and monitoring subsystem, which uses a programmable logic controller, manages the BPC subsystems.

Description: Advantages and disadvantages of synthetic soils are discussed. It is pointed out that synthetic soils may provide the proper physical and chemical properties necessary to maximize plant growth, such as a toxic-free composition and cation exchange capacities. The importance of nutrient retention, aeration, moisture retention, and mechanical support as qualities for synthetic soils are stressed. Zeoponics, or the cultivation of plants in zeolite substrates that both contain essential plant-growth cations on their exchange sites and have minor amounts of mineral phases and/or anion-exchange resins that supply essential plant growth ions, is discussed. It is suggested that synthetic zeolites at lunar bases could provide adsorption media for separation of various gases, act as catalysts and as molecular sieves, and serve as cation exchangers in sewage-effluent treatment, radioactive-waste disposal, and pollution control. A flow chart of a potential zeoponics system illustrates this process.

Description: This paper investigates the efficiency of low- and high-atomic number materials used as protective shields against biologically effective radiation in doses equivalent to those expected in low-earth-orbit and interplanetary manned missions. Results are presented on calculations for single-material shields from polyethylene, water, Be, Al, Fe, and Ta and multilayer shelds made from the combinations of any two or any three of these materials, for both LEO and interplanetary conditions. It is shown that, whereas for protons and Galactic cosmic rays the ordering of shield materials has a negligible effect, for electrons and secondary bremsstrahlung, both the order and the composition are important parameters. It was found that low-atomic-number materials are most effective shields against protons and galactic cosmic rays, and are most effective in decreasing bremsstrahlung production, while high-atomic-number shields are the best attenuators of both primary electrons (if the dose is dominated by primary electrons) and secondary bremsstrahlung (if this is produced).

Description: A wavelength that lies within a spectral interval of reduced solar emission (a Fraunhofer line) can carry optical communications with reduced interference from direct or reflected background sunlight. Suitable Fraunhofer lines are located within the tuning range of good candidate lasers. The laser should be tunable dynamically to track Doppler shifts in the sunlight incident on any solar system body that may appear in the background as viewed by the receiver. A Fraunhofer filter used with a direct-detection receiver should be tuned to match the Doppler shifts of the source and background. The required tuning calculated here for various situations is also required if, instead, one uses a heterodyne receiver with limited post-detection bandwidth.

Description: NASA research on the functioning of biological systems in space and the resulting improvement in space suits and life-support systems is discussed. The centrifuge facility which will provide the major elements of a life science research facility for Spacelab and Space Station Freedom is described, and the Vestibular Research Facility for studying motion sickness in space is examined. The aluminum AX-5 space suit and the Controlled Ecological Life-Support System are described, and biomedical sensors for studying bone mass changes in space are discussed. Studies on the use of exercise to control tissue-fluid shifts in space are described.

Description: The authors investigate microwave imaging of metallic objects using a diversity method and interpret and predict the reconstructed image from an approach based on analysis of the scattering mechanism and a back-projection algorithm used in image retrieval. The connection between the various scattering mechanisms and the reconstructed images is discussed, what the images represent is interpreted, and a prediction is made as to what the image will look like over given spectral and angular windows. A brief description is given of the microwave diversity imaging system and the formulation of the microwave diversity imaging based on the physical optics approximation. The scattering mechanism of a complex shaped metallic object is then briefly reviewed and an alternate approach to interpreting the reconstructed image based on the understanding of the scattering mechanism and the reconstruction algorithm is given. Several numerical and experimental examples are included to support this interpretation approach.

Description: An examination is presented of three techniques used for the efficient computation of fields diffracted by a subreflector that has been shaped by geometrical optics synthesis. It is found that these techniques, which are based on the geometrical theory of diffraction (GTD), produce errors in the computed fields that are specific to shaped reflectors. These errors are examined for a reflector system shaped to produce maximum gain from a tapered feed illumination. The discrepancies are directly related to the caustic being located near an observation point of the GTD calculations. The errors found are localized, and they increase in magnitude as the caustic approaches the main reflector. In a general offset geometry, the location of the caustic may be located arbitrarily close to the main reflector given a prescribed output aperture distribution. For the specific case considered here-the common situation of shaping to produce maximum gain-the caustic is located near the edge of the main reflector and on the reflection shadow boundary. A local correction is derived which creates a uniform solution through the caustic and across the reflection shadow boundary. Away from this point the calculation receeds to the standard GTD solution.

Description: The effect of filtering on the signal-to-noise ratio (SNR) of a coherently demodulated band-limited signal is determined in the presence of worst-case amplitude ripple. The problem is formulated as an optimizaton in the Hilbert space L2. The form of the worst-case amplitude ripple is specified, and the degradation in the SNR is derived in closed form. It is shown that, when the maximum passband amplitude ripple is 2Delta (peak-to-peak), the SNR is degraded by at most (1-Delta-squared), even when the ripple is unknown or uncompensated. For example, an SNR loss of less than 0.01 dB due to amplitude ripple can be assured by keeping the amplitude ripple under 0.42 dB.

Description: Radar cross section (RCS) reduction by absorber covering is experimentally studied by employing microwave diversity imaging. Experimental results show that broadband absorber covering is not effective at reducing the co-polarized (the transmitting and receiving antennas have opposite sense of circular polarization) RCS of a plate when the incident wave approximates the edge-on direction but is effective at reducing the cross-polarized (both the transmitting and receiving antennas have the same sense of circular polarization) RCS for all incident directions. The surface current absorber covering is effective at reducing the nonspecular energy and multiple bounces regardless of the polarization status of the measurement.

Description: The gain of circularly polarized (CP) array antennas realized by proper phasing of sequentially rotated linearly polarized (LP) elements is compared to that of arrays using CP elements and demonstrated by calculations for microstrip patch elements. When element spacing is large and array size is small, the advantages of LP elements are offset by the significant reduction in gain due to high cross polarized lobes in the diagonal planes. For large arrays of closely spaced elements, this gain loss reduces to a negligible amount. However, for spacings above a critical value of about 0.7 wavelengths, unacceptably high gain losses will be incurred.

Description: The use of an imaging radar polarimeter data for unsupervised classification of scattering behavior is described by comparing the polarization properties of each pixel in a image to that of simple classes of scattering such as even number of reflections, odd number of reflections, and diffuse scattering. For example, when this algorithm is applied to data acquired over the San Francisco Bay area in California, it classifies scattering by the ocean as being similar to that predicted by the class of odd number of reflections, scattering by the urban area as being similar to that predicted by the class of even number of reflections, and scattering by the Golden Gate Park as being similar to that predicted by the diffuse scattering class. It also classifies the scattering by a lighthouse in the ocean and boats on the ocean surface as being similar to that predicted by the even number of reflections class, making it easy to identify these objects against the background of the surrounding ocean. The algorithm is also applied to forested areas and shows that scattering from clear-cut areas and agricultural fields is mostly similar to that predicted by the odd number of reflections class, while the scattering from tree-covered areas generally is classified as being a mixture of pixels exhibiting the characteristics of all three classes, although each pixel is identified with only a single class.

Description: The electromagnetic properties of a structure that is both chiral and periodic are investigated using coupled-mode equations. The periodicity is described by a sinusoidal perturbation of the permittivity, permeability, and chiral admittance. The coupled-mode equations are derived from physical considerations and used to examine bandgap structure and reflected and transmitted fields. Chirality is observed predominantly in transmission, whereas periodicity is present in both reflection and transmission.

Description: A method is presented for analyzing a finite planar array of circular microstrip patches fed by coaxial probes. The self- and mutual impedances between array elements are calculated using the method of moments with the dyadic Green's function for a dielectric layer on a ground plane. The patch circuits are determined by using the reaction integral equation. The active input impedance as well as the active element pattern of the array are computed from a knowledge of the resultant patch currents. The calculated results for two-element and eight-element linear arrays are in good agreement with experimental data. The active reflection coefficient and element pattern for the center and edge elements of a two-dimensional array as a function of scan angle are also presented.

Description: The issue of reducing the cost of phased array vehicle antennas through the use of a lens feeding arrangement instead of phase shifters at each element is addressed. In particular, the economic viability of a mobile satellite system (MSAT) is largely dependent on the efficient use of the allocated scarce spectrum and orbit as well as the satellite power. the type of vehicle antenna used will play a critical role in achieving this efficiency. A standard design approach for an electronically steered array uses phase shifters at each element to provide beam steering. A method for reducing the required number of phase shifters by using an R-KR lens feed network is outlined. The authors briefly discuss the phase shifter approach to beam steering, examine various lens feed techniques, and describe the R-KR lens approach. The lens feed network architecture is examined, a computer model for simulation of the array is presented, and the results of analysis of a suggested design for the MSAT application are given. In addition, satellite acquisition and tracking considerations are investigated.

Description: Slot timing recovery in a direct-detection optical PPM communication system can be achieved by processing the photodetector output waveform with a nonlinear device whose output forms the input to a phase-locked loop. The choice of a simple transition detector as the nonlinearity is shown to give satisfactory synchronization performance. The rms phase error of the recovered slot clock and the effect of slot timing jitter on the bit error probability were directly measured. The experimental system consisted of an AlGaAs laser diode (wavelength = 834 nm) and a silicon avalanche photodiode photodetector. The system used Q = 4 PPM signaling and operated at a source data rate of 25 Mbits/s. The mathematical model developed to compute the rms phase error of the recovered clock is shown to be in good agreement with results of actual measurements of phase errors. The use of the recovered slot clock in the receiver resulted in no significant degradation in receiver sensitivity compared to a system with perfect slot timing. The system achieved a bit error probability of 10 to the -6th at a received optical signal energy of 55 detected photons per information bit.

Description: The moment method solution to the problem of a reactively loaded circular patch is presented. Using the reaction integral equation in conjuction with the method of moments, parameters of the Thevenin's equivalent network for the loaded patch are obtained. From the equivalent network parameters an expression for the imput impedance of the loaded patch is derived. A design procedure for a circularly polarized disk antenna is presented. Computed results are compared with the experimental data.

Description: The use of commercially available 1-bit LSI correlator chips whose main function is the correlation of sequences of plus ones and minus ones (rather than zeros and ones) as building blocks in the design of multibit correlators is investigated. A radix-2 number system in which the bit values are + or - 1 (no zero) is discussed. The development of a simple conversion algorithm to find the + or - 1 representation of a number given in 2's complement is presented. Some of the issues raised by this conversion algorithm are discussed. It is shown that, if the analog signal is available, there is a simple step to take before the analog-to-digital conversion in order to improve the overall precision. The construction of multibit correlators based on the + or - 1 system is compared to alternative approaches.

Description: The operation and evaluation of a bioreactor designed for high intensity oxygen transfer in a microgravity environment is described. The reactor itself consists of a zero headspace liquid phase separated from the air supply by a long length of silicone rubber tubing through which the oxygen diffuses in and the carbon dioxide diffuses out. Mass transfer studies show that the oxygen is film diffusion controlled both externally and internally to the tubing and not by diffusion across the tube walls. Methods of upgrading the design to eliminate these resistances are proposed. Cell growth was obtained in the fermenter using Saccharomyces cerevisiae showing that this concept is capable of sustaining cell growth in the terrestrial simulation.

Description: A mixture of ammonium hydroxide with acetic acid and a slurry of human feces, urine, and wipes were used as CELSS model wastes to be wet-oxidized at temperatures from 250 to 500 C, i.e. below and above the critical point of water (374 C and 218 kg/sq cm or 21.4 MPa). The effects of oxidation temperature ( 250-500 C) and residence time (0-120 mn) on carbon and nitrogen and on metal corrosion from the reactor material were studied. Almost all of the organic matter in the model wastes was oxidized in the temperature range from 400 to 500 C, above the critical conditions for water. In contrast, only a small portion of the organic matter was oxidized at subcritical conditions. A substantial amount of nitrogen remained in solution in the form of ammonia at temperatures ranging from 350 to 450 C suggesting that, around 400 C, organic carbon is completely oxidized and most of the nitrogen is retained in solution. The Hastelloy C-276 alloy reactor corroded during subcritical and supercritical water oxidation.

Description: The production rate and solid content of waste streams found in a life support system for a space habitat (in which plants are grown for food) are discussed. Two recycling scenarios, derived from qualitative considerations as opposed to quantitative mass and energy balances, tradeoff studies, etc., are presented; they reflect differing emphases on and responses to the waste stream formation rates and their composition, as well as indicate the required products from waste treatment that are needed in a life support system. The data presented demonstrate the magnitude of the challenge to developing a life support system for a space habitat requiring a high degree of closure.

Description: Research and technology development issues centering on the recycling of materials within a bioregenerative life support system are reviewed. The importance of recovering waste materials for subsequent use is emphasized. Such material reclamation will substantially decrease the energy penalty paid for bioregenerative life support systems, and can potentially decrease the size of the system and its power demands by a significant amount. Reclamation of fixed nitrogen and the sugars in cellulosic materials is discussed.

Description: The photosynthesis and productivity of Lemna gibba is analyzed for CELSS based plant growth. Net photosynthesis of Lemna gibba is determined as a function of incident photosynthetic photon flux (PPF), with the light coming from above, below, or from both directions. Light from below is about 75 percent as effective as from above when the stand is sparse, but much less so with dense stands. High rates of photosynthesis are measured at 750 micromol / sq m per sec PPF and 1500 micromol/ mol CO2 at densities up to 660 g fresh weight (FW)/ sq m with young cultures. The analysis includes diagrams illustrating the net photosynthesis response to bilateral lighting of a sparse stand of low assimilate Lemna gibba; the effect of stand density on the net photosynthesis response to bilateral lighting of high assimilate Lemna gibba; the net photosynthesis response to ambient CO2 of sparse stands of Lemna gibba; and the time course of net photosynthesis and respiration per unit chamber and per unit dry weight of Lemna gibba.

Description: Several activities are performed related to the definition and creation of telerobotic systems. The effort and investment required to create architectures for these complex systems can be enormous; however, the magnitude of process can be reduced if structured design techniques are applied. A number of informal methodologies supporting certain aspects of the design process are available. More recently, prototypes of integrated tools supporting all phases of system design from requirements analysis to code generation and hardware layout have begun to appear. Activities related to system architecture of telerobots are described, including current activities which are designed to provide a methodology for the comparison and quantitative analysis of alternative system architectures.

Description: Two types of precedence relationship representations for mechanical assembly sequences are presented: precedence relationships between the establishment of one connection between two parts and the establishment of another connection, and precedence relationships between the establishment of one connection and states of the assembly process. Precedence relationship representations have the advantage of being very compact. The problem with these representations was how to guarantee their correctness and completeness. Two theorems are presented each of which leads to the generation of one type of precedence relationship representation guaranteeing its correctness and completeness for a class of assemblies.

Description: The analysis and development of a manipulator redundant in structure and sensor devices controlled by a distributed multiprocessor architecture are discussed. The goal has been the realization of a modular structure of the manipulator with evident aspects of flexibility and transportability. The distributed control structure, thanks to his modularity and flexibility could be integrated in the future into an operative structure aimed to space telerobotics. The architecture is applied to the 6 DOF manipulator Gilberto.

Description: The control of a multifingered hand slave in order to accurately exert arbitrary forces and impart small movements to a grasped object is, at present, a knotty problem in teleoperation. Although a number of articulated robotic hands have been proposed in the recent past for dexterous manipulation in autonomous robots, the possible use of such hands as slaves in teleoperated manipulation is hindered by the present lack of sensors in those hands, and (even if those sensors were available) by the inherent difficulty of transmitting to the master operator the complex sensations elicited by such sensors at the slave level. An analysis of different problems related to sensor-based telemanipulation is presented. The general sensory systems requirements for dexterous slave manipulators are pointed out and the description of a practical sensory system set-up for the developed robotic system is presented. The problem of feeding back to the human master operator stimuli that can be interpreted by his central nervous system as originated during real dexterous manipulation is then considered. Finally, some preliminary work aimed at developing an instrumented glove designed purposely for commanding the master operation and incorporating Kevlar tendons and tension sensors, is discussed.

Description: Reflected-force feedback is an important aspect of teleoperations. The objective is to determine the ability of the human operator to respond to that force. Telerobotics operation is simulated by computer control of a motor-driven device with capabilities for programmable force feedback and force measurement. A computer-controlled motor drive is developed that provides forces against the fingers as well as (angular) position control. A load cell moves in a circular arc as it is pushed by a finger and measures reaction forces on the finger. The force exerted by the finger on the load cell and the angular position are digitized and recorded as a function of time by the computer. Flexure forces of the index, long and ring fingers of the human hand in opposition to the motor driven load cell are investigated. Results of the following experiments are presented: (1) Exertion of maximum finger force as a function of angle; (2) Exertion of target finger force against a computer controlled force; and (3) Test of the ability to move to a target force against a force that is a function of position. Averaged over ten individuals, the maximum force that could be exerted by the index or long finger is about 50 Newtons, while that of the ring finger is about 40 Newtons. From the tests of the ability of a subject to exert a target force, it was concluded that reflected-force feedback can be achieved with the direct kinesthetic perception of force without the use of tactile or visual clues.

Description: After a brief tutorial on the NASA/National Bureau of Standards Standard Reference Model for Telerobot Control System Architecture (NASREM) functional architecture, the approach to its implementation is shown. First, interfaces must be defined which are capable of supporting the known algorithms. This is illustrated by considering the interfaces required for the SERVO level of the NASREM functional architecture. After interface definition, the specific computer architecture for the implementation must be determined. This choice is obviously technology dependent. An example illustrating one possible mapping of the NASREM functional architecture to a particular set of computers which implements it is shown. The result of choosing the NASREM functional architecture is that it provides a technology independent paradigm which can be mapped into a technology dependent implementation capable of evolving with technology in the laboratory and in space.

Description: Much of the technology planned for use in NASA's Flight Telerobotic Servicer (FTS) and the Demonstration Test Flight (DTF) is relatively new and untested. To provide the answers needed to design safe, reliable, and fully functional robotics for flight, NASA/GSFC is developing a robotics technology testbed for research of issues such as zero-g robot control, dual arm teleoperation, simulations, and hierarchical control using a high level programming language. The testbed will be used to investigate these high risk technologies required for the FTS and DTF projects. The robotics technology testbed is centered around the dual arm teleoperation of a pair of 7 degree-of-freedom (DOF) manipulators, each with their own 6-DOF mini-master hand controllers. Several levels of safety are implemented using the control processor, a separate watchdog computer, and other low level features. High speed input/output ports allow the control processor to interface to a simulation workstation: all or part of the testbed hardware can be used in real time dynamic simulation of the testbed operations, allowing a quick and safe means for testing new control strategies. The NASA/National Bureau of Standards Standard Reference Model for Telerobot Control System Architecture (NASREM) hierarchical control scheme, is being used as the reference standard for system design. All software developed for the testbed, excluding some of simulation workstation software, is being developed in Ada. The testbed is being developed in phases. The first phase, which is nearing completion, and highlights future developments is described.

Description: A study was conducted to develop a preliminary definition of the Flight Telerobotic Servicer (FTS) that could be used to understand the operational concepts and scenarios for the FTS. Called the Tinman, this design concept was also used to begin the process of establishing resources and interfaces for the FTS on Space Station Freedom, the National Space Transportation System shuttle orbiter, and the Orbital Maneuvering vehicle. Starting with an analysis of the requirements and task capabilities as stated in the Phase B study requirements document, the study identified eight major design drivers for the FTS. Each of these design drivers and their impacts on the Tinman design concept are described. Next, the planning that is currently underway for providing resources for the FTS on Space Station Freedom is discussed, including up to 2000 W of peak power, up to four color video channels, and command and data rates up to 500 kbps between the telerobot and the control station. Finally, an example is presented to show how the Tinman design concept was used to analyze task scenarios and explore the operational capabilities of the FTS. A structured methodology using a standard terminology consistent with the NASA/National Bureau of Standards Standard Reference Model for Telerobot Control System Architecture (NASREM) was developed for this analysis.

Description: Key to an efficient accomplishment of space station servicing operations is the development of a scenario where the presence of man in space is well integrated with the capability of teleoperated and automatic robot system outside the stations. Results focusing on mission requirements, trajectory sequences, propulsion subsystem features, and manipulative kit characteristics relevant to proximity servicing during a Man Tended Free Flyers Robotic Mission (MTFF-RM) are illustrated.

Description: An integration of 3-D vision systems with robot manipulators will allow robots to operate in a poorly structured environment by visually locating targets and obstacles. However, by using computer vision for objects acquisition makes the problem of overall system calibration even more difficult. Indeed, in a CAD based manipulation a control architecture has to find an accurate mapping between the 3-D Euclidean work space and a robot configuration space (joint angles). If a stereo vision is involved, then one needs to map a pair of 2-D video images directly into the robot configuration space. Neural Network approach aside, a common solution to this problem is to calibrate vision and manipulator independently, and then tie them via common mapping into the task space. In other words, both vision and robot refer to some common Absolute Euclidean Coordinate Frame via their individual mappings. This approach has two major difficulties. First a vision system has to be calibrated over the total work space. And second, the absolute frame, which is usually quite arbitrary, has to be the same with a high degree of precision for both robot and vision subsystem calibrations. The use of computer vision to allow robust fine motion manipulation in a poorly structured world which is currently in progress is described along with the preliminary results and encountered problems.

Description: Telerobotics studies remote control of distant robots by a human operator using supervisory or direct control. Even if the robot manipulators has vision or other senses, problems arise involving control, communications, and delay. The communication delays that may be expected with telerobots working in space stations while being controlled from an Earth lab have led to a number of experiments attempting to circumvent the problem. This delay in communication is a main motivating factor in moving from well understood instantaneous hands-on manual control to less well understood supervisory control; the ultimate step would be the realization of a fully autonomous robot. The 3-D model control plays a crucial role in resolving many conflicting image processing problems that are inherent in resolving in the bottom-up approach of most current machine vision processes. The 3-D model control approach is also capable of providing the necessary visual feedback information for both the control algorithms and for the human operator.

Description: An adaptive control algorithm is proposed for a class of nonlinear systems, such as robotic manipulators, which is capable of improving its performance in repetitive motions. When the task is repeated, the error between the desired trajectory and that of the system is guaranteed to decrease. The design is based on the combination of a direct adaptive control and a learning process. This method does not require any knowledge of the dynamic parameters of the system.

Description: The Flight Telerobotic Servicer (FTS) program will require an ability to develop, in a cost effective manner, many simulation models for design, analysis, performance evaluation, and crew training. Computational speed and the degree of modeling fidelity associated with each simulation must be commensurate with problem objectives. To demonstrate evolving state-of-the-art general purpose multibody modeling capabilities, to validate these by laboratory testing, and to expose their modeling shortcomings, two focus problems at the opposite ends of the simulation spectrum are defined: (1) Coarse Acquisition Control Dynamics. Create a real time man-in-the-control-loop simulator. Provide animated graphical display of robot arm dynamics and tactile feedback sufficient for cueing the operator. Interface simulator software with human operated tactile feedback controller; i.e., the Kraft mini-master. (2) Fine, Precision Mode Control Dynamics. Create a high speed, high fidelity simulation model for the design, analysis, and performance evaluation of autonomous 7 degree-of-freedom (DOF) trajectory control algorithms. This model must contain detail dynamic models for all significant dynamics elements within the robot arm, such as joint drive mechanisms.

Description: The Flight Telerobotic Servicer (FTS) is being developed by the Goddard Space Flight Center (GSFC) for performing a variety of assembly, servicing, inspection and maintenance tasks on the Space Station. The Project Office at GSFC has tasked the Engineering Directorate to assemble a robotics research and development program which will support the FTS project. The activities center around support for the Development Test Flight (DTF) on the Space Shuttle and investigations of operational problems associated with the FTS on Space Station Freedom. For the DTF, areas such as control algorithms, safety systems, and end-effectors will be developed. For FTS operations, the emphasis will be to develop a dual-arm bi-lateral force-reflecting teleoperator and use it as an FTS Operational Simulator (FTSOS). The simulator will be used to investigate operational techniques, camera configurations, operator interfacing, orbital replacement unit (ORU) designs, end-effector designs, and training techniques. After a series of test activities, reports will be generated for input to the DTF and FTS designs.

Description: The definition of manipulability ellipsoids for dual robot systems is given. A suitable kineto-static formulation for dual cooperative robots is adopted which allows for a global task space description of external and internal forces, and relative velocities. The well known concepts of force and velocity manipulability ellipsoids for a single robot are formally extended and the contributions of the two single robots to the cooperative system ellipsoids are illustrated. Duality properties are discussed. A practical case study is developed.

Description: The manipulator design discussed here results from the examination of some of the reasons why redundancy is necessary in general purpose manipulation systems. A spherical joint design actuated in-parallel, having the many advantages of parallel actuation, is described. In addition, the benefits of using redundant actuators are discussed and illustrated in the design by the elimination of loci of singularities from the usable workspace with the addition of only one actuator. Finally, what is known by the authors about space robotics requirements is summarized and the relevance of the proposed design matched against these requirements. The design problems outlined here are viewed as much from the mechanical engineering aspect as from concerns arising from the control and the programming of manipulators.

Description: The presence of redundant degrees of freedom in a manipulator structure leads to a physical phenomenon known as a self-motion, which is a continuous motion of the manipulator joints that leaves the end-effector motionless. In the first part of the paper, a global manifold mapping reformulation of manipulator kinematics is reviewed, and the inverse kinematic solution for redundant manipulators is developed in terms of self-motion manifolds. Global characterizations of the self-motion manifolds in terms of their number, geometry, homotopy class, and null space are reviewed using examples. Much previous work in redundant manipulator control has been concerned with the redundancy resolution problem, in which methods are developed to determine, or resolve, the motion of the joints in order to achieve end-effector trajectory control while optimizing additional objective functions. Redundancy resolution problems can be equivalently posed as the control of self-motions. Alternatives for redundancy resolution are briefly discussed.

Description: The U.S. Bureau of Mines is approaching the problems of accidents and efficiency in the mining industry through the application of automation and robotics to mining systems. This technology can increase safety by removing workers from hazardous areas of the mines or from performing hazardous tasks. The short-term goal of the Automation and Robotics program is to develop technology that can be implemented in the form of an autonomous mining machine using current continuous mining machine equipment. In the longer term, the goal is to conduct research that will lead to new intelligent mining systems that capitalize on the capabilities of robotics. The Bureau of Mines Automation and Robotics program has been structured to produce the technology required for the short- and long-term goals. The short-term goal of application of automation and robotics to an existing mining machine, resulting in autonomous operation, is expected to be accomplished within five years. Key technology elements required for an autonomous continuous mining machine are well underway and include machine navigation systems, coal-rock interface detectors, machine condition monitoring, and intelligent computer systems. The Bureau of Mines program is described, including status of key technology elements for an autonomous continuous mining machine, the program schedule, and future work. Although the program is directed toward underground mining, much of the technology being developed may have applications for space systems or mining on the Moon or other planets.

Description: The approach for arriving at design guidelines for assembly by robots in outerspace is illustrated. The use of robots in a zero gravity environment necessitates that extra factors over and above normal design guidelines be taken into account. Besides, many of the guidelines for assembly by robots on earth do not apply in space. However, considering the axioms for normal design and assembly as one set, guidelines for design and robotic assembly as another, and guidelines for design and assembly in space as the third set, unions and intersections of these sets can generate guidelines for two or more of these conditions taken together - say design and manual assembly in space. Therein lies the potential to develop expert systems in the future, which would use an exhaustive database and similar guidelines to arrive at those required by a superposition of these conditions.

Description: Most robotic systems today are designed one at a time, at a high cost of time and money. This wasteful approach has been necessary because the industry has not established a foundation for the continued evolution of intelligent machines. The next generation of robots will have to be generic, versatile machines capable of absorbing new technology rapidly and economically. This approach is demonstrated in the success of the personal computer, which can be upgraded or expanded with new software and hardware at virtually every level. Modularity is perceived as a major opportunity to reduce the 6 to 7 year design cycle time now required for new robotic manipulators, greatly increasing the breadth and speed of diffusion of robotic systems in manufacturing. Modularity and its crucial role in the next generation of intelligent machines are the focus of interest. The main advantages that modularity provides are examined; types of modules needed to create a generic robot are discussed. Structural modules designed by the robotics group at the University of Texas at Austin are examined to demonstrate the advantages of modular design.

Description: An experimental study was conducted to determine the effects of various forms of visual and force feedback on human performance for several telemanipulation tasks. Experiments were conducted with varying frame rates and subtended visual angles, with and without force feedback.

Description: Data describing the microsurgeon's hand dynamics was recorded and analyzed in order to provide an accurate model for the telemicrosurgery application of the Bimanual Telemicro-operation Test Bed. The model, in turn, will guide the development of algorithms for the control of robotic systems in bimanual telemicro-operation tasks. Measurements were made at the hand-tool interface and include position, acceleration and force between the tool-finger interface. Position information was captured using an orthogonal pulsed magnetic field positioning system resulting in measurements in all six degrees-of-freedom (DOF). Acceleration data at the hands was obtained using accelerometers positioned in a triaxial arrangement on the back of the hand allowing measurements in all three cartesian-coordinate axes. Force data was obtained by using miniature load cells positioned between the tool and the finger and included those forces experienced perpendicular to the tool shaft and those transferred from the tool-tissue site. Position data will provide a minimum/maximum reference frame for the robotic system's work space or envelope. Acceleration data will define the response times needed by the robotic system in order to emulate and subsequently outperform the human operator's tool movements. The force measurements will aid in designing a force-reflective, force-scaling system as well as defining the range of forces the robotic system will encounter. All analog data was acquired by a 16-channel analog-to-digital conversion system residing in a IBM PC/AT-compatible computer at the Center's laboratory. The same system was also used to analyze and present the data.

Description: A new and simple approach to configuration control of redundant manipulators is presented. In this approach, the redundancy is utilized to control the manipulator configuration directly in task space, where the task will be performed. A number of kinematic functions are defined to reflect the desirable configuration that will be achieved for a given end-effector position. The user-defined kinematic functions and the end-effector Cartesian coordinates are combined to form a set of task-related configuration variables as generalized coordinates for the manipulator. An adaptive scheme is then utilized to globally control the configuration variables so as to achieve tracking of some desired reference trajectories. This accomplishes the basic task of desired end-effector motion, while utilizing the redundancy to achieve any additional task through the desired time variation of the kinematic functions. The control law is simple and computationally very fast, and does not require the complex manipulator dynamic model.

Description: A frequent complaint in the computer oriented trade journals is that current hardware technology is progressing so quickly that software developers cannot keep up. A example of this phenomenon can be seen in the field of microcomputer graphics. To exploit the advantages of new mechanisms of information storage and retrieval, new approaches must be made towards incorporating existing programs as well as developing entirely new applications. A particular area of need is the correlation of discrete image elements to textural information. The interactive digital video (IDV) interface embodies a new concept in software design which addresses these needs. The IDV interface is a patented device and language independent process for identifying image features on a digital video display and which allows a number of different processes to be keyed to that identification. Its capabilities include the correlation of discrete image elements to relevant text information and the correlation of these image features to other images as well as to program control mechanisms. Sophisticated interrelationships can be set up between images, text, and program control mechanisms.

Description: Medical illustration is a field of visual communication with a long history. Traditional medical illustrations are static, 2-D, printed images; highly realistic depictions of the gross morphology of anatomical structures. Today medicine requires the visualization of structures and processes that have never before been seen. Complex 3-D spatial relationships require interpretation from 2-D diagnostic imagery. Pictures that move in real time have become clinical and research tools for physicians. Medical illustrators are involved with the development of interactive visual displays for three different, but not discrete, functions: as educational materials, as clinical and research tools, and as data bases of standard imagery used to produce visuals. The production of interactive displays in the medical arts is examined.

Description: One of the most remarkable perceptual properties of common experience is that the perceived shapes of known objects are constant despite movements about them which transform their projections on the retina. This perceptual ability is one aspect of shape constancy (Thouless, 1931; Metzger, 1953; Borresen and Lichte, 1962). It requires that the viewer be able to sense and discount his or her relative position and orientation with respect to a viewed object. This discounting of relative position may be derived directly from the ranging information provided from stereopsis, from motion parallax, from vestibularly sensed rotation and translation, or from corollary information associated with voluntary movement. It is argued that: (1) errors in exocentric judgements of the azimuth of a target generated on an electronic perspective display are not viewpoint-independent, but are influenced by the specific geometry of their perspective projection; (2) elimination of binocular conflict by replacing electronic displays with actual scenes eliminates a previously reported equidistance tendency in azimuth error, but the viewpoint dependence remains; (3) the pattern of exocentrically judged azimuth error in real scenes viewed with a viewing direction depressed 22 deg and rotated + or - 22 deg with respect to a reference direction could not be explained by overestimation of the depression angle, i.e., a slant overestimation.

Description: Pictorial displays whose primary purpose is to convey accurate information about the 3-D spatial layout of an environment are discussed. How and how well, pictures can convey such information is discussed. It is suggested that picture perception is not best approached as a unitary, indivisible process. Rather, it is a complex process depending on multiple, partially redundant, interacting sources of visual information for both the real surface of the picture and the virtual space beyond. Each picture must be assessed for the particular information that it makes available. This will determine how accurately the virtual space represented by the picture is seen, as well as how it is distorted when seen from the wrong viewpoint.

Description: Systematic errors in perception and memory present a challenge to theories of perception and memory and to applied psychologists interested in overcoming them as well. A number of systematic errors in memory for maps and graphs are reviewed, and they are accounted for by an analysis of the perceptual processing presumed to occur in comprehension of maps and graphs. Visual stimuli, like verbal stimuli, are organized in comprehension and memory. For visual stimuli, the organization is a consequence of perceptual processing, which is bottom-up or data-driven in its earlier stages, but top-down and affected by conceptual knowledge later on. Segregation of figure from ground is an early process, and figure recognition later; for both, symmetry is a rapidly detected and ecologically valid cue. Once isolated, figures are organized relative to one another and relative to a frame of reference. Both perceptual (e.g., salience) and conceptual factors (e.g., significance) seem likely to affect selection of a reference frame. Consistent with the analysis, subjects perceived and remembered curves in graphs and rivers in maps as more symmetric than they actually were. Symmetry, useful for detecting and recognizing figures, distorts map and graph figures alike. Top-down processes also seem to operate in that calling attention to the symmetry vs. asymmetry of a slightly asymmetric curve yielded memory errors in the direction of the description. Conceptual frame of reference effects were demonstrated in memory for lines embedded in graphs. In earlier work, the orientation of map figures was distorted in memory toward horizontal or vertical. In recent work, graph lines, but not map lines, were remembered as closer to an imaginary 45 deg line than they had been. Reference frames are determined by both perceptual and conceptual factors, leading to selection of the canonical axes as a reference frame in maps, but selection of the imaginary 45 deg as a reference frame in graphs.

Description: The status of motion as a minimal information source for perceiving the environmental properties of surface segregation, three-dimensional (3-D) form, displacement, and dynamics is discussed. The selection of these particular properties was motivated by a desire to present research on perceiving properties that span the range of dimensional complexity.

Description: The classical notion of how things are seen is that perception is passive, that the eyes are windows, and in floods reality. Physiological work of the 19th century cast doubt on this view that perception is passive acceptance of reality. Perception is not at the present time a popular topic for philosophers. This must be partly because scientific accounts of perception have now gone a long way away from appearances. They depend on physiological and psycho-physical experiments which require technical investigation and do not fall within traditional concepts of philosophy. Theories of visual perception are examined, both from a physical and psycho-physical standpoint.

Description: Recent development in video technology, such as the liquid crystal displays and shutters, have made it feasible to incorporate stereoscopic depth into the 3-D representations on 2-D displays. However, depth has already been vividly portrayed in video displays without stereopsis using the classical artists' depth cues described by Helmholtz (1866) and the dynamic depth cues described in detail by Ittleson (1952). Successful static depth cues include overlap, size, linear perspective, texture gradients, and shading. Effective dynamic cues include looming (Regan and Beverly, 1979) and motion parallax (Rogers and Graham, 1982). Stereoscopic depth is superior to the monocular distance cues under certain circumstances. It is most useful at portraying depth intervals as small as 5 to 10 arc secs. For this reason it is extremely useful in user-video interactions such as telepresence. Objects can be manipulated in 3-D space, for example, while a person who controls the operations views a virtual image of the manipulated object on a remote 2-D video display. Stereopsis also provides structure and form information in camouflaged surfaces such as tree foliage. Motion parallax also reveals form; however, without other monocular cues such as overlap, motion parallax can yield an ambiguous perception. For example, a turning sphere, portrayed as solid by parallax can appear to rotate either leftward or rightward. However, only one direction of rotation is perceived when stereo-depth is included. If the scene is static, then stereopsis is the principal cue for revealing the camouflaged surface structure. Finally, dynamic stereopsis provides information about the direction of motion in depth (Regan and Beverly, 1979). Clearly there are many spatial constraints, including spatial frequency content, retinal eccentricity, exposure duration, target spacing, and disparity gradient, which - when properly adjusted - can greatly enhance stereodepth in video displays.

Description: Observers frequently underestimate the in-depth slant of rectangles under reduction conditions. This also occurs for slanted rectangles depicted on a flat display medium. Perrone (1982) provides a model for judged slant based upon properties of the 2-D trapezoidal projection of the rectangle. Two important parameters of this model are the angle of convergence of the sides of the trapezoid and the projected length of the trapezoid. This model was tested using a range of stimulus rectangles and found that the model failed to predict some of the major trends in the data. However, when the projected width of the base of the trapezoid projection was used in the model, instead of the projected length, excellent agreement between the theoretical and obtained slant judgements resulted. The good fit between the experimental data and the new model predictions indicates that perceived slant estimates are highly correlated with specifiable features in the stimulus display.

Description: Limited cue, open-loop tasks in which a human observer indicates distances or relations among distances are discussed. By open-loop tasks, it is meant tasks in which the observer gets no feedback as to the accuracy of the responses. What happens when cues are added and when the loop is closed are considered. The implications of this research for the effectiveness of visual displays is discussed. Errors in visual distance tasks do not necessarily mean that the percept is in error. The error could arise in transformations that intervene between the percept and the response. It is argued that the percept is in error. It is also argued that there exist post-perceptual transformations that may contribute to the error or be modified by feedback to correct for the error.

Description: Experiences raise a number of concerns for future spatial-display developers. While the promise of spatial displays is great, the cost of their development will be correspondingly large. The knowledge and skills which must be coordinated to ensure successful results is unprecedent. From the viewpoint of the designer, basic knowledge of how human beings perceive and process complex displays appears fragmented and largely unquantified. Methodologies for display development require prototyping and testing with subject pilots for even small changes. Useful characterizations of the range of differences between individual users is nonexistent or at best poorly understood. The nature, significance, and frequency of interpretation errors associated with complex integrated displays is unexplored and undocumented territory. Graphic displays have intuitive appeal and can achieve face validity much more readily than earlier symbolic displays. The risk of misleading the pilot is correspondingly greater. Thus while some in the research community are developing the tools and techniques necessary for effective spatial-display development, potential users must be educated about the issues so that informed choices can be made. The scope of the task facing all is great. The task is challenging and the potential for meaningful contributions at all levels is high indeed.

Description: In each of two studies, subjects were exposed to a continuously changing prismatic displacement with a mean value of 19 prism diopters (variable displacement) and to a fixed 19-diopter displacement (fixed displacement). In Experiment 1, significant adaptation (post-pre shifts in hand-eye coordination) was found for fixed, but not for variable, displacement. Experiment 2 demonstrated that adaptation was obtained for variable displacement, but it was very fragile and is lost if the measures of adaptation are preceded by even a very brief exposure of the hand to normal or near-normal vision. Contrary to the results of some previous studies, an increase in within-S dispersion was not found of target pointing responses as a result of exposure to variable displacement.

Description: Displays are now being used extensively throughout the society. More and more time is spent watching television, movies, computer screens, etc. Furthermore, in an increasing number of cases, the observer interacts with the display and plays the role of operator as well as observer. To a large extent, the normal behavior in the normal environment can also be thought of in these same terms. Taking liberties with Shakespeare, it might be said, all the world's a display and all the individuals in it are operators in and on the display. Within this general context of interactive display systems, a discussion is began with a conceptual overview of a particular class of such systems, namely, teleoperator systems. The notion is considered of telepresence and the factors that limit telepresence, including decorrelation between the: (1) motor output of the teleoperator as sensed directly via the kinesthetic/tactual system, and (2) the motor output of the teleoperator as sensed indirectly via feedback from the slave robot, i.e., via a visual display of the motor actions of the slave robot. Finally, the deleterious effect of time delay (a particular decorrelation) on sensory-motor adaptation (an important phenomenon related to telepresence) is examined.

Description: The user interface of a computer system is a visual display that provides information about the status of operations on data within the computer and control options to the user that enable adjustments to these operations. From the very beginning of computer technology the user interface was a spatial display, although its spatial features were not necessarily complex or explicitly recognized by the users. All text and nonverbal signs appeared in a virtual space generally thought of as a single flat plane of symbols. Current technology of high performance workstations permits any element of the display to appear as dynamic, multicolor, 3-D signs in a virtual 3-D space. The complexity of appearance and the user's interaction with the display provide significant challenges to the graphic designer of current and future user interfaces. In particular, spatial depiction provides many opportunities for effective communication of objects, structures, processes, navigation, selection, and manipulation. Issues are presented that are relevant to the graphic designer seeking to optimize the user interface's spatial attributes for effective visual communication.

Description: For better or worse, virtual imaging displays are with us in the form of narrow-angle combining-glass presentations, head-up displays (HUD), and head-mounted projections of wide-angle sensor-generated or computer-animated imagery (HMD). All military and civil aviation services and a large number of aerospace companies are involved in one way or another in a frantic competition to develop the best virtual imaging display system. The success or failure of major weapon systems hangs in the balance, and billions of dollars in potential business are at stake. Because of the degree to which national defense is committed to the perfection of virtual imaging displays, a brief consideration of their status, an investigation and analysis of their problems, and a search for realistic alternatives are long overdue.

Description: Spatial displays, and a constraint that they do not place on the use of spatial instruments are discussed. Much of the work done in visual perception by psychologists and by computer scientists has concerned displays that show the motion of rigid objects. Typically, if one assumes that objects are rigid, one can then proceed to understand how the constant shape of the object can be perceived (or computed) as it moves through space. The author maintains that photographs and cinema are visual displays that are also powerful forms of art. Their efficacy, in part, stems from the fact that, although viewpoint is constrained when composing them, it is not nearly so constrained when viewing them. It is obvious, according to the author, that human visual systems did not evolve to watch movies or look at photographs. Thus, what photographs and movies present must be allowed in the rule-governed system under which vision evolved. Machine-vision algorithms, to be applicable to human vision, should show the same types of tolerance.

Description: Converging evidence from several sources indicates that two distinct representations of visual space mediate perception and visually guided behavior, respectively. The two maps of visual space follow different rules; spatial values in either one can be biased without affecting the other. Ordinarily the two maps give equivalent responses because both are veridically in register with the world; special techniques are required to pull them apart. One such technique is saccadic suppression: small target displacements during saccadic eye movements are not preceived, though the displacements can change eye movements or pointing to the target. A second way to separate cognitive and motor-oriented maps is with induced motion: a slowly moving frame will make a fixed target appear to drift in the opposite direction, while motor behavior toward the target is unchanged. The same result occurs with stroboscopic induced motion, where the frame jump abruptly and the target seems to jump in the opposite direction. A third method of separating cognitive and motor maps, requiring no motion of target, background or eye, is the Roelofs effect: a target surrounded by an off-center rectangular frame will appear to be off-center in the direction opposite the frame. Again the effect influences perception, but in half of the subjects it does not influence pointing to the target. This experience also reveals more characteristics of the maps and their interactions with one another, the motor map apparently has little or no memory, and must be fed from the biased cognitive map if an enforced delay occurs between stimulus presentation and motor response. In designing spatial displays, the results mean that what you see isn't necessarily what you get. Displays must be designed with either perception or visually guided behavior in mind.

Description: The extent to which perceptual judgements within egocentric and exocentric frames of reference are subject to illusory disturbances and long term modifications is discussed. It is argued that well known spatial illusions, such as the oculogyral illusion and induced visual motion have usually been discussed without proper attention being paid to the frame of reference within which they occur, and that this has led to the construction of inadequate theories and inappropriate procedures for testing them.

Description: Principles for the design of dynamic spatial instruments for communicating quantitative information to viewers are considered through a brief review of the history of pictorial communication. Pictorial communication is seen to have two directions: (1) from the picture to the viewer; and (2) from the viewer to the picture. Optimization of the design of interactive instruments using pictorial formats requires an understanding of the manipulative, perceptual, and cognitive limitations of human viewers.

Description: It is appropriate to note that 1988 marks the 100th anniversary of the birth of the reflector antenna. It was in 1888 that Heinrich Hertz constructed the first one, a parabolic cylinder made of sheet zinc bent to shape and supported by a wooden frame. Hertz demonstrated the existence of the electromagnetic waves that had been predicted theoretically by James Clerk Maxwell some 22 years earlier. In the 100 years since Hertz's pioneering work the field of electromagnetics has grown explosively: one of the technologies is that of remote sensing of planet Earth by means of electromagnetic waves, using both passive and active sensors located on an Earth Science Geostationary Platform (ESEP). For these purposes some exquisitely sensitive instruments were developed, capable of reaching to the fringes of the known universe, and relying on large reflector antennas to collect the minute signals and direct them to appropriate receiving devices. These antennas are electrically large, with diameters of 3000 to 10,000 wavelengths and with gains approaching 80 to 90 dB. Some of the reflector antennas proposed for ESGP are also electrically large. For example, at 220 GHz a 4-meter reflector is nearly 3000 wavelengths in diameter, and is electrically quite comparable with a number of the millimeter wave radiotelescopes that are being built around the world. Its surface must meet stringent requirements on rms smoothness, and ability to resist deformation. Here, however, the environmental forces at work are different. There are no varying forces due to wind and gravity, but inertial forces due to mechanical scanning must be reckoned with. With this form of beam scanning, minimizing momentum transfer to the space platform is a problem that demands an answer. Finally, reflector surface distortion due to thermal gradients caused by the solar flux probably represents the most challenging problem to be solved if these Large Space Antennas are to achieve the gain and resolution required of them.

Description: During a period spanning more than 5 years, low elevation radar measurements of rain were systematically obtained in the mid-Atlantic coast of the U.S. Drop size distribution measurements with a disdrometer were also acquired on the same rain days. The drop size data were utilized to convert the radar reflectivity factors to estimated rain rates for the respective rain days of operation. Applying high level algorithms to the rain data, core values of rain intensities were identified (peak rain rates), and families of rain rate isopleths analyzed. In particular, equicircle diameters of the family of isopleths enveloping peak rain intensities were statistically characterized. The presented results represents the analysis of two rain days, 12 radar scans, corresponding to 430 culled rain rate isopleths from an available data base of 22,000 contours, approximately 100 scans encompassing 17 rain days. The results presented show trends of the average rain rate vs. contour scale dimensions, and cumulative distributions of rain cell dimensions which belong to core families of precipitation.

Description: Propagation effects in the aeronautical mobile-satellite service differ from those in the fixed-satellite service and other mobile-satellite services because: small antennas are used on aircraft, and the aircraft body may affect the performance of the antenna; high aircraft speeds cause large Doppler spreads; aircraft terminals must accommodate a large dynamic range in transmission and reception; and due to their high speeds, banking maneuvers, and three-dimensional operation, aircraft routinely require exceptionally high integrity of communications, making even short-term propagation effects very important. Data and models specifically required to characterize the path impairments are discussed, which include: tropospheric effects, including gaseous attenuation, cloud and rain attenuation, fog attenuation, refraction and scintillation; surface reflection (multipath) effects; ionospheric effects such as scintillation; and environmental effects (aircraft motion, sea state, land surface type). Aeronautical mobile-satellite systems may operate on a worldwide basis, including propagation paths at low elevation angles. Several measurements of multipath parameters over land and sea were conducted. In some cases, laboratory simulations are used to compare measured data and verify model parameters. The received signals is considered in terms of its possible components: a direct wave subject to atmospheric effects, and a reflected wave, which generally contains mostly a diffuse component.

Description: In 1992, an Engineering Test Satellite 6 is scheduled to be launched by an H-2 rocket. The missions of ETS-6 are to establish basic technologies of inter-satellite communications using S-band, millimeter waves and optical beams and of fixed and mobile satellite communications using multibeam antenna on board the satellite. A plan of the experiments is introduced.

Description: The JPL Channel Simulator lab was modified to allow full duplex links and to allow the use of field propagation data for link fading. This capability will be used to test equipment for the joint AUSSAT/NASA mobile satellite experiment in July 1989.

Description: Propagation experiments for maritime, aeronautical, and land mobile satellite communications were performed using Engineering Test Satellite-Five (ETS-5). The propagation experiments are one of major mission of Experimental Mobile Satellite System (EMSS) which is aimed for establishing basic technology for future general mobile satellite communication systems. A brief introduction is presented for the experimental results on propagation problems of ETS-5/EMSS.

Description: Land-mobile satellite propagation measurements were implemented at L Band (1.5 GHz) in South-Eastern Australia during an 11 day period in October 1988. Transmissions (CW) from both the Japanese ETS-5 and INMARSAT Pacific geostationary satellites were accessed. Previous measurements in this series were performed at both L Band (1.5 GHz) and UHF (870 MHz) in Central Maryland, North-Central Colorado, and the southern United States. The objectives of the Australian campaign were to expand the data base acquired in the U.S. to another continent, to validate a U.S. derived empirical model for estimating the fade distribution, to establish the effects of directive antennas, to assess the isolation between co- and cross-polarized transmissions, to derive estimates of fade as well as non-fade durations, and to evaluate diversity reception. All these objectives were met.

Description: Results from the most recent MSAT-X field experiments, the Tower-3 Experiment and the JPL/FAA/INMARSAT MARECS-B2 Satellite Experiment, are presented. Results that distinguish the propagation environment of the tower set-up are given and explained. The configuration and flight variables of the aeronautical experiment which used an FAA aircraft and an INMARSAT satellite are described. Results that highlight the disturbances on the aeronautical satellite channel are presented. The roles of satellite-induced signal variations and of multipatch are identified and their impact on the link is discussed.

Description: Future communication systems will move toward the 20 to 30 GHz frequency range for wider bandwidth and reduced interference, and will use small earth terminals. The ESA satellite OLYMPUS is scheduled for launch this June and will be ready for use in October. OLYMPUS has 12, 20, and 30 GHz beacons. Virginia Tech and Michigan Tech are working with NASA/JPL on an OLYMPUS experiment and hardware development program. OLYMPUS beacons provide coverage of the east coast of the US sufficient for attenuation measurements. The planned hardware for the OLYMPUS experiments is illustrated. The components of the experiment are summarized. A block diagram level overview of a typical channel is given and one of the four RF front ends is shown.

Description: ACTS counters fading by resource sharing between the users. It provides a large margin only for those terminals which are at risk by unfavorable atmospheric conditions. ACTS, as an experimental satellite, provides a 5 dB clear weather margin and 10 dB additional margin via rate reduction and encoding. For the uplink, this margin may be increased by exercising uplink power control. Some of the challenges faced by the radiowave propagation community are listed. The issue of needs for the satellite are listed, both general and specific.

Description: Some information is given on the ITALSAT millimetric waves propagation experiment, which is to be conducted with the ITALSAT satellite, whose launch is foreseen for the middle of 1990. The purpose of the experiment is one of experimenting with advanced technologies and techniques employing the 20/30 GHz bands in wideband telecommunications. Among the most qualified features of this system are the multispot antenna and the exchange function performed directly onboard. Details of the experiment are given.

Description: Ground based radiometer measurements at 20.6, 31.65, and 90.0 GHz were analyzed to provide attenuation statistics, thus extending the data base of previous NAPEX studies. Using data from colocated radiosondes, comparisons of measurements and calculations of brightness temperatures are presented. The oxygen absorption model of Rosenkranz and the water vapor absorption models of Liebe and of Waters are used. Data from July 1987 at San Nicolas Island, California and from December 1987, August and November 1988 at Denver, Colorado, are included. Joint attenuation statistics at 20.6 and 31.65 GHz are presented for two locations of the Colorado Research Network for December 1987 and August 1988.

Description: The need to develop accurate models for secondary statistics of fading land mobile satellite signals has motivated a study of fading signal autocorrelations and multipath spectrum. Results of autocorrelations and power spectral densities from measured data are presented and comparisons to multipath spectrum models are made.

Description: Beam-waveguide (BWG) antennas provide multiple frequency band operations and other operational benefits for large ground-station antennas. Present design practices use diffraction analyses that ignore the presence of the BWG enclosure and may be inaccurate at lower frequency bands for ground-station antennas operating over multiple frequency bands. Introduced here is a new analysis approach that considers the presence of the BWG enclosure. Results based on the new analysis have revealed new understandings of the performance degradation mechanisms in a BWG antenna and have provided direction for potential design improvements.

Description: The benefits of improved radiometric tracking data have been studied for planetary approach within the inner Solar System using the Mars Rover Sample Return trajectory as a model. It was found that the benefit of improved data to approach and encounter navigation was highly dependent on the a priori uncertainties assumed for several non-estimated parameters, including those for frame-tie, Earth orientation, troposphere delay, and station locations. With these errors at their current levels, navigational performance was found to be insensitive to enhancements in data accuracy. However, when expected improvements in these errors are modeled, performance with current-accuracy data significantly improves, with substantial further improvements possible with enhancements in data accuracy.

Description: It is demonstrated how the use of interleaving/deinterleaving in trellis-coded modulation (TCM) systems can reduce the signal-to-noise ratio loss due to imperfect carrier demodulation references. Both the discrete carrier (phase-locked loop) and suppressed carrier (Costas loop) cases are considered and the differences between the two are clearly demonstrated by numerical results. These results are of great importance for future communication links to the Deep Space Network (DSN), especially from high Earth orbiters, which may be bandwidth limited.

Description: Thermal measurements of microwave transmitter feedhorn windows were performed using an imaging infrared radiometer. The measurement technique is described and results are presented for windows made of 0.001-in. Kapton (trademark of Dupont Chemical Co.) and 0.1-in. HTP-6 (Space Shuttle tile material). Measured and calculated temperatures agree well.

Description: The design and testing of the prototype horn for the proposed 1-megawatt radar are described. The unique features of this square horn include a multiflare design in which flare angle changes rather than corrugations are used to generate the required higher-order modes. A five-port combining section is used at the input. The design of this section and the multiflare section are described. Measured radiation patterns are in good agreement with theoretical patterns.

Description: Aperture efficiency measurements made during 1988 on the three 70-m stations (DSS-14, DSS-43, and DSS-63) at X-band (8420 MHz) and S-band (2295 MHz) have been analyzed and reduced to yield best estimates of antenna gain versus elevation. The analysis has been carried out by fitting the gain data to a theoretical expression based on the Ruze formula. Newly derived flux density and source-size correction factors for the natural radio calibration sources used in the measurements have been used in the reduction of the data. Peak gains measured at the three stations were 74.18 (plus or minus 0.10) dBi at X-band, and 63.34 (plus or minus 0.03) dBi at S-band, with corresponding peak aperture efficiencies of 0.687 (plus or minus 0.015) and 0.762 (plus or minus 0.006), respectively. The values quoted assume no atmosphere is present, and the estimated absolute accuracy of the gain measurements is approximately plus or minus 0.2 dB at X-band and plus or minus 0.1 dB at S-band (1-sigma values).

Description: The characteristics of the Synthetic Aperture Radar (SAR) antenna aboard the ESA Remote Sensing Satellite (ERS-1) are presented. The antenna is folded into a dense package for launch and is deployed in orbit. The design requirements and constraints, their impact on the design, and the resulting features of the mechanisms are discussed.

Description: The various formulations of Maxwell's equations are reviewed with emphasis on those formulations which most readily form analogies with Navier's equations. Analogies involving scalar and vector potentials and electric and magnetic field components are presented. Formulations allowing for media with dielectric and conducting properties are emphasized. It is demonstrated that many problems in electromagnetism can be solved using the NASTRAN finite element code. Several fundamental problems involving time harmonic solutions of Maxwell's equations with known analytic solutions are solved using NASTRAN to demonstrate convergence and mesh requirements. Mesh requirements are studied as a function of frequency, conductivity, and dielectric properties. Applications in both low frequency and high frequency are highlighted. The low frequency problems demonstrate the ability to solve problems involving media inhomogeneity and unbounded domains. The high frequency applications demonstrate the ability to handle problems with large boundary to wavelength ratios.

Description: The objective of any radar experiment is to determine as much as possible about the entities which scatter the radiation. This review discusses many of the various parameters which can be deduced in a radar experiment, and also critically examines the procedures used to deduce them. Methods for determining the mean wind velocity, the RMS fluctuating velocities, turbulence parameters, and the shapes of the scatterers are considered. Complications with these determinations are discussed. It is seen throughout that a detailed understanding of the shape and cause of the scatterers is important in order to make better determinations of these various quantities. Finally, some other parameters, which are less easily acquired, are considered. For example, it is noted that momentum fluxes due to buoyancy waves and turbulence can be determined, and on occasions radars can be used to determine stratospheric diffusion coefficients and even temperature profiles in the atmosphere.

Description: Fundamental methods of signal processing used in normal mesosphere stratosphere troposphere (MST) radar observations are described. Complex time series of received signals obtained in each range gate are converted into Doppler spectra, from which the mean Doppler shift, spectral width and signal-to-noise ratio (SNR) are estimated. These spectral parameters are further utilized to study characteristics of scatterers and atmospheric motions.